Natural gas is frequently transported by pipeline from a supply source to a distant market. It is oftentimes desirable to operate the pipeline under a substantially constant and high load factor. However, at times the deliverability or capacity of the pipeline may exceed demand while at other times the demand may exceed the deliverability or capacity of the pipeline. In order to shave off peaks when demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store excess gas in such a manner that it can be delivered during periods when demand exceeds supply. Such practice allows future demand peaks to be met with stored natural gas. One practical means for doing this is to convert natural gas into a liquefied state such as liquefied natural gas (“LNG”) via a liquefaction process for storage during periods of low demand and then vaporize the liquefied natural gas as demand requires. Liquefaction of natural gas can be especially useful when a pipeline is either not available or impractical for transporting natural gas from a supply source that is separated by a great distance to a candidate market. Moreover, transport of natural gas by ocean-going vessels is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
An example of a liquefaction technique is cryogenic liquefaction which can reduce the volume of the natural gas up to about 600-fold. Cryogenic liquefaction can convert natural gas into liquefied natural gas that can be stored and transported at near atmospheric pressures. Cryogenic liquefaction process can involve cooling natural gas down to about −240° F. to about −260° F. while the liquefied natural gas is at near-atmospheric vapor pressure. Natural gas is liquefied by sequentially passing the natural gas at an elevated pressure through a plurality of cooling stages whereupon the natural gas is cooled to successively lower temperatures until liquefaction temperature is reached. Cooling may be accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combination of the preceding refrigerants (i.e., mixed refrigerant systems). Some liquefaction techniques employ an open methane cycle for the final refrigeration cycle where a pressurized LNG-bearing stream is flashed. The flash vapors (i.e., the flash gas stream(s)) are subsequently used as cooling agents, recompressed, cooled, combined with processed natural gas feed stream. The combined stream may then be liquefied to produce a pressurized LNG-bearing stream.
One technical challenge that can arise during liquefaction of natural gas is the removal of heavy hydrocarbons. While natural gas is primarily comprised of methane, it may also contain heavy hydrocarbon components. These heavy hydrocarbon components should be removed from the natural gas prior to liquefaction since heavy hydrocarbon components can freeze and/or foul downstream heat exchangers. To avoid these potential issues, LNG facilities can include one or more heavies removal columns for removing heavy hydrocarbon components. However, conventional heavies removal columns often require operation within very narrow ranges of temperature, pressure, and feed composition in order to efficiently remove heavy hydrocarbon components. In some cases, a variation of a few degrees in feed temperature of a conventional heavies removal column can cause all or most of the fluid in the column to turn to liquid, which can result in major process upsets. Moreover, incorporation of heavies removal columns in a liquefaction system can increase power requirements of subsequent refrigeration systems (e.g., ethylene refrigeration system). In some cases, these power requirements can substantially limit operation of a liquefaction system. Thus, a need exists for a process and an apparatus employing a heavies removal column that can reduce the power requirements of subsequent refrigeration systems.